Airplane landing wheel brake control apparatus



Dec. 22, 1953 G. w. YARBER 2,663,521

AIRPLANE LANDING WHEEL BRAKE CONTROL APPARATUS Filed April 2o, 1951 AT1-@EN EYS Patented Dee. 22, i953 UNITED STATES E T OFF I CE AIRPLANE LANDING WHEEL'BRAKE CONTROL' APPARATUS This invention relates to automatic brake control systems and more particularly to automatic skid-preventing apparatus for airplane' landing` wheel brake systems. The present invention is' directed to certain improvements over the genn erally' similar automatic systemsr disclosed in my copending application Serial No. 90,965, iiled May 2, 1949, now U. S. Patent No. 2,631,709 of March 17, i952, and in the-copending application Serial No. 145,368, now U. S. Patent No. 2,636,700 of AApril 2S, 1953, led February 2l, l95'0`,.of Gordon W. Yarber, Harry H. Howell and Rush F. Chase. The presently improved system is herein illustratively described by reference to its application in the. case of large bomber, transport or similar airplanes. The various constants o1' the circuit apparatus, including the different time delay iactors and the sizes or magnitudes ofY the diierent circuit parameters are given as representative, in a typical systemdevelopedA forthat purpose.

A general object of Vthis invention isto provide an improved and more versatile anti-skid brake control system of the type described, capable ofA operating under a Wider range of airplane4 landing conditions and' of coping automatically with a greater number of different events or contingencies than previous systems. As a result the improved system herein describedl provides more reliable and efiicient brake operation,v and overcomes to an even greater extent theV possible eiiects of human error and misjudgment of the pilot in controlling the brakes than' heretofore.

More specifically, an object of the presentinvention is to provide an improved locked-wheel prevention control, which will insure thatl the' landing'wheel brakes cannot be applied untilY the landing Wheelhas accelerated substantially to full rotative speed upon initialground contactthereof. The purposeef such control, therefore. istoprevent landing with a skidding Wheel;y as such a skid might go undetected, andi moreover, even if it were detected oy themechanismmight initiate a degenerative brakingr condition. cir the system, from which it might not. readily recover.

A related object of the inventionisa lockedwheel' prevention control' which Will automaticalf ly become' inoperative when. the airplane is; brought to aY standstill, so^ that the` lockedlwhe'el' prevention control will not be permitted to release the' arplanesA brakes at a time when it is desired to maintain them in the applied condition.

Still another object is a locked wheel prevention control which will operate effectively during bounce landings as well as` during the initial appreach of the' airplane'toV the ground. As a result such control will prevent untimely application ofthe brakes in the interval between successive ground contacts ofthe Wheel in the case of a faulty or bounce landing, as desired.

l Another objectorv the invention is to provide a locked Wheel prevention control operable to preventv locked Wheel condition during landing on an extremely slippery runway, such as a run- Way covered with wet ice, for example. if, for instance, braking eiilcie'ncy of the system should degenerate' progressively to the low point represented by the locked wheel condition, said locked-wheel -preventionlcontrol will automatically release the brakes long enough to permit ac'- celeration ci the wheel and thereby initiate a l process of regeneration of braking eiiiciency.

- pilot;

In its preferred and herein illustrated forni, the invention is applied as an automatic control system which at certain times and under certain conditions selectively interrupts and restores braking otherwise under control of the airplane In thefexample. such results are accomplished by selective'. energization and deenergization oi a solenoid valve: interposed in 'the hydraulic pressure= conduit to a Wheels brakes. However, the invention isA not necessarily limited in these respects as will become evident from an understanding of. the generalnature thereof set forth herein.

In generaLtherefore, the invention comprises brakel-releasemeans operable in one sense or direction to eiiect release ofthe brakes and in an other sense' to' effect application or reapplication thereof,v combined' with various automatic' In' other Words, such automatic system means in the system controlling operation of such brake-release means. The principal or central portion of the automatic means in the sysu tem comprises skid-preventing means which, as in the above-cited two patent applications, automatically regulates application of the brakes to obtain maximum eiective braking effort with minimum skidding of the wheel. Preferably the automatic skid-preventing means is of the type generally disclosed in the above-cited application Serial No. 90,965, although systems embodying features of the present invention are not necessarily limited thereto.

An important feature of the present invention resides in the provision of locked-wheel prevent ing means controlled by wheel rotation and operatively connected to the brake-release means for actuating the same and thereby eifecting ref lease of the brake automatically in response to stopping of wheel rotation. Such locked-wheel preventing means is arranged to operate independently of the automatic skid-preventing apparatus.

More specific features of the invention reside in the particular apparatus comprising such lockedwheel preventing means. As herein described, the locked wheel condition is prevented by a locked-wheel control relay and an energizing circuit therefor which includes a wheel-rotated commutator or switch and suitable circuit means such as a full-wave bridge rectifier operable to convert commutator signals into control relay energizing current during rotation of the wheel. When the braked wheel stops rotating, the resulting release of the locked-wheel control relay causes actuation or energization of the brakerelease means to free such wheel so that it may again rotate. When the wheel again rotates, the locked-wheel control relay is reenergized, a development which might be expected to restore braking and thereby the original locked wheel condition. However, time delay means included in the locked-wheel control relay is again energized by such renewal of wheel rotation. Thus under exceedingly slippery runway conditions, should wheel braking degenerate to the locked wheel condition by operation of the skid-preventing portion of the system, the brakes will be automatically released for a definite short period of time which is usually long enough for initiation of a regenerative action restoring the wheel to running speed and the braking operations to normal, as more fully explained hereinafter.

Another feature of the improved system resides in the provision therein of arming means operable to disable the locked-wheel preventing means with the airplane at a standstill, so that the brakes may then be locked without interference, as desired. However, in order to insure that said arming or disabling means will remain inoperative under other conditions, such as during the landing run in general, and thereby will not interfere with normal operations of the lockedwheel preventing means, such arming means is itself deactivated by and during normal operation of the skid-preventing means. Because the normal operation of the skid-preventing means is intermittent by nature, a time delay element is provided with such arming means to extend the period of effectiveness thereof sufficiently to Vbridge the intervals between successive operation of the skid-preventing means, so that such arming means will not become ineffectual duringy 4 normal intervals between operations of such skid-preventing means.

Still another feature of the improved system resides in the provision of an independent control for the locked-wheel preventing means which enables it to be used as a means for the additional purpose of insuring that the brakes will be in released condition during approach of the airplane to the ground, so that the brakes may not be applied before landing or prematurely upon landing. Such independent control preferably comprises a Weight actuated device such as an electrical switch on the oleo mechanism of the landing gear which, while the airplane is in the air and the oleo is extended, renders the disabling means inoperative and thereby insures that the locked-wheel preventing means may function for its additional purpose just mentioned. The same control element cooperates with the locked-wheel preventing means to produce a similar result in the event of a bounce landing so that the brakes will not be locked upon the second ground con tact of the landing wheel at the end of a bounce. These and other features, objects and advantages of the present invention will become more fully evident from the following description of the preferred application thereof illustrated in the accompanying drawing. The drawing is a schematic diagram of an automatic control system illustrating the present invention as well as certain features or components more fully illustrated and described in the above cited copending patent applications.

General description The system illustrated in the circuit diagram is that which is used for controlling the brakes of one landing wheel of the airplane, the solenoid valve l5 being connected in the hydraulic conduits to control the brake. Apart from certain elements common to all, a similar system and associated solenoid valve controlled thereby will be used for each additional wheel of the airplane to be provided with automatically controlled braking.

Any conventional solenoid valve of suitable form may be used which will block flow of pressure uid from conduit l2 to the wheel brake (not shown) through conduit I2' when the control solenoid I is energized, and which will automatically restore communication between such vconduits when such solenoid is deenergiaed.

During the energized condition of solenoid Ill the valve interconnects conduits I2 and i4, the latter returning to the prime source of pressure fiuid to permit relief of pressure in the brake actuators (not shown). The function of the system appearing in the figure is to energize and deenergize solenoid i0' in the manner and during the times and conditions to be described more fully hereinafter, to control removal and Y reapplication of the brakes accordingly.

In an airplane, the voltage applied to conductor I6 (upper right hand corner of diagram) will ordinarily be in the range from 18 to 30 volts D. C. A master control switch i8 is closed when the system is to be energized. Closure of such l switch applies voltage to the bus conductor 2t having several branches in the circuit. Similar conductors 20a, 2Gb and 20c are provided to carry Y' An indicator light 22 in- Y terposed in one of the branches of conductor 20 f 5. in series with contact Zb ofv relay Z illuminates when this relay assumes released condition. This relay is termed a fail-safe relay and is of the slow release type. As later explained, release of relay Z occurs only when the system has failedto operate within a normal or prescribed period corresponding to the slow release period of the relay, because of malfunctioning of some part or section of the system. Under such conditions release of the relay Z deenergizes solenoid I and permits reapplication of the brakes despite the operating conditions in any other part of the sys.- tem. The indicator light 22 then informs the pilot that the system has failed whereby he will know that the brakes must be operated solely by manual control, or without, benefit of the automatic system.

A landing gear switch 24 is mounted on the landing wheel oleo mechanism in such manner that when the` wheel is oli thel ground and the oleo is extended, switch 24 will be closed. Thus switch 24 is a normally closed switch arranged to be opened automatically by Contact of the landing wheel with the ground to displace the oleo mechanism. In ilight. this switch applies voltage to the distribution conductor 26 having several branches as shown. Similar conductors 26a, 26h and 2do carry voltage to the other similar brake control systems, to which landing gear switch 24 is common. Conductor 26 is connected I through rectier 28 to conductor 3l? which interconnects the upper side of the relay Z and the uppermost contact Xd of relay X. The purpose of rectifier 28 is merely to prevent reverse application of energy to conductor 2t, namely from conductor 2t, through relay contact Xd and conductor 33 when the landing gear switch 24 is open.

The lower side of relay Z is connected to ground conductor 34 through a conductor 32. A variable resistance 3% and a condenser 38 are connected in series between conductors 30 andV 32, that is, across the coil of relay Z. The purpose oi this R-C combination is to extend the natural release period of slow release relay Z to a value oi about 2.75 seconds, although the value is not critical and in a typical case may be permitted to vary as a result of manufacturing tolerances from 2.0 seconds to 3.5 seconds, for example. The length of this delay period of relay Z is selected to span the longest delayed operating condition in the remaining portions of the system under normal conditions, so that if such other portions of the system fail to operate within the selected period, then it will be because of some extreme or abnormal condition not met by the system or because of malfunctioning of the system. The fail-safe relay Z will then release to restore braking, as previously mentioned.

Under normal operating conditions relay Z is therefore energized and its upper switch arm en,- gages the relay contact Zc while its lower switch arm engages the dead contact Za. The contact Zc is connected to the contact Xcl of relay X through conductor 40. Contact Xc is engaged by upper arm of relay X when the latter is energized and thereby forms an energizing circuit for the solenoid valve solenoid l0 through conductor 40. Thus relay X is properly termed a solenoid valve control relay. The lower switch arm of relay X is adapted to engage upper relay Lit conductor 42 to the upperk side of; the winding; Qf: relay L. as Shown. ySinclar conductors 4.24,. .4112 and 42e extend from; the corresponding contactsA ofA relays corresponding to relay X in the other similar automatic control systems for the remaining braked wheels of the airplane.

Closure of relay contact Xa forms anl energizing circuit for relay L, through conductor 4,2 and conductor 47. This is the only energizing circuit; for relay L when the airplane is on the. ground., as landing gear switch 2 4 is thenopen. In flight., with landing gear switch 24 closed, relay L` is; continuously energized through conductor 26, reef. tier 44, a section of conductor 42 andV conductor 4l. The purpose of rectifier 44 is to preventreverse application of voltage from conductor 4,2. to conductor 23, during energization of relay- X. The lower side of relay L and the lower contactLa; ci this relay are grounded at 48. A variable re,- sistor 49 and a condenser 52 in series therewith are connected between conductors 4.2 and 4.1: to.- provide an extended slow release period for relay' L of about 1.5 seconds. The slow release period of this relay is not critical and in av typical,

' case may vary between one and two seconds, for

example.

Relay L may be termed an arming relay, and that portion of the circuit surrounded by dotted line 5l, an arming relay circuit. This. term is; intended to indicate that the purpose of relay L, is to arm the system to insure that the landing wheel brake will be released under certain conditions or that it will not be released undercertain other conditions, as will be explained? hereinafter.

The switch arm ci relay L is connected through conductor 54 to the switch arm of relay V which 1 is termed a locked wheel control relay. Similar conductors 54h, and 54e extend to corresponding relays (V)l in the corresponding similar automatic control systems of .the airplane. Theupper contact Vd of relay V isy engaged during the released condition of the relay and therebyV establishes an energizing circuit forthe relay W through rectifier 64, conductor 56, conductor 54y and relay contact La, assuming relay L is then energized. A condenser 53 is connected between conductor 56 and the upper side of relay W- to provide an extended slow release period forthis relay following interruption of such energizing circuit, such slow release period being about 0.6 second. However, the presence of rectier 64 between the lower side of relay W and` conductor 53 prevents condenser Se from extending the release period of relay W when the energizing cur,-V rent through the latter is interrupted during iiow through a separate energizing circuit for this relay, including conductor t6, as will appear here,- inafter.

A separate energizing circuit for relay W and including conductor t@ can be formed by closure ofA skid contacts S of slzid-and-reeovery switch mechanism Sl constructed and arranged to oper-V ate inV accordance with the disclosure in my co. pending application Serial No. 90,965 filed May 2, 1949. Closure ci such skid. contacts completesI an energizing circuit for relay W throughy con.` ductor 53 and ground conducto-r 52. Still an-v other separate encrgizing circuit for relay W and including a diierent branch of conductor may be formed by closure of` recovery contacts R of, switch mechanismv 6l. In .this case the energize ingv .circuit includesl the conductor m4 andthel upper contact Yb of relay Y assuming the latter is in released condition. Relay Y is termed a recovery control relay and is of the slow release type, as will be explained more fully later herein. As disclosed in my ,earlier application Serial No. 90,5l65, mentioned above, the wheel skid-andrecovery switch mechanism el includes a nywheel and an overrunning clutch or slipping coupling rotatively interconnecting such flywheel and the landing wheel. When the landing wheel accelerates at the beginning of the landing run,.the flywheel is spun. rl'ordue required to spin the flywheel actuates the recovery contacts R. When the landing wheel decelerates upon application of brakes and the flywheel overruns the landing wheel through the slip-ping coupling, the skid contacts S are actuated by a reverse torque. When acceleration of the landing Wheel commences at a speed materially below the existing speed of the flywheel, such as will usually be the case following interruption Iof a skid by release of the brake the sliid contacts S will remain closed until the landing wheel accelerates to a speed about equal to the instantaneous speed of the flywheel and applies torque to the latter to increase its speed, thereby closing the recovery contacts R. The specific details of construction oi the switch mechanism 5i may be obtained from said copending application, it being sufficient for present purposes to understand the foregoing general nature operation or such mechanism. In accordance With the presently improved system a rotative switch or coinrnutator is mechanically coupled to the landing wheel to be rotated directly thereby. Such commutator consists oi an insulating body portion, in the periphery of which four conductive segments lo are placed to be engaged successively by the sliding brush contact cil occupying a fixed position in the landing gear structure, indicated by dotted linesv l2. The contacts 'lil are interconnected and are grounded through conductor t2.

The sliding brush contact t8 is connected through conductor lll to the corner 84% or a bridge rectifier circuit. Voltage from supply conductor 253 is applied to such corner or the rectiiier circuit through the voltage-dropping resistors Se and et. The :opposite corner t5 or such rectifier is connected to ground conductor Se through storage condenser t8. The upper corner Ql of such rectifier is connected to the upper side of relay V, while the lower corner is connected tothe lower side of such relay. The four rectiiier elements it, it, Se and t2 in the respective branches of the rectifier bridge are so arranged in polarity that a surge or" current iiows through relay V into condenser et when an insulating portion of connnutator initially engages brush contact while another surge of current flows through relay V when brush et is initially engaged by a conductive segment 'it of such commutator.

, The latter current is the result or" discharge from storage condenser E8. Thus as commutator E5 continues to rotate, relay V receives a pulsating direct current which maintains energization thereof. The circuit constants a typical case are such that rotation of the four-segment comn mutator t5 at a speed above about twenty-one revolutions per minute maintains relay V steadily energized. At a slower speed of commutator rotation the relay armature is caused to open and close intermittently at a frequency dependent onsuch speed of rotation. Gf course, increasing the number of commutator segments would reduce the speed of 2l R. P. M. to a proportionately n greater detail.

3 lower value, as would increasing the slow release period of relay V.

The only purpose of condenser 95 is to remove transient oscillations in the energizing pulses of current in relay V, which oscillations were found to exist otherwise when the wave form was examined on an oscilloscope. Such condenser could be omitted from the circuit, however, without seriously impairing its operation.

The function of relay V, cooperating with commutator E35, the bridge rectifier circuit and arming relay L is to prevent locked Wheel co-ndition beiore ground contact of the landing wheel and to prevent release oi the brakes when the airplane is at a standstill, as later explained in When the commutator tt ceases to rotate altogether, no current will flow in the winding of relay V. This is true if the brush t8 rests on a conductive segment it of the commutator, as Well as when it engages the insulating body portion of such commutator, as storage condenser functions as a direct-current blocking condenser, allowing only one impulse of current to iloy/ through relay V between succes-A sive switching actions oi conirnutator @5. Relay V may properly be termed a locked-wheel control relay. Relay V is of the slow release type, having a slow r lease period of about 0.37 secn ond, although in a typical case this period may vary from 0.25 to 9.50 second, for instance, without causing difficulty.

Relay Y, which may be termed a recovery control relay, is connected to relay contact Wb through conductors El and 98, and is thereby energized during the released condition of relay W, which is normally about 0.16 second, but may vary between 0.12 and 0.20 second without causing difficulty. The slow release period of relay Y is extended to about 0.35 second by the variable resistor itu and 'the condenser |82 connected in series across the winding of the relay. Again, the slow release period of relay Y may vary between 03 and 0.1i second without appreciably altering the operation of the system.

Relay X, which may be termed the solenoid valve control relay, is energized alternately to relay Y by relay W, the skid control relay. Thus energization of relay W produces energization of relay X through relay contact Wc, resistor Hi8, conductor itt, conductor H0 and resistor H2. The purpose of resistor HBS is to limit short-circuit current owing through conductor Idd, relay contact Ya and conductor lilll when recovery contacts R. close during the energized or unreleased condition of relay Y. The purpose of resistor H2 connected in series with relay X is to effectively increase the latters resistance during such short circuit condition, so that the short circuit may be effective to ground out relay X by closure of contacts R. The purpose of condenser iifl by-passine resistor H2 is to reduce the impedance of the energizing circuit for relay X upon energiaation or relay W to engage its contact Wa, so that relay X will possess sui cient magnetization to draw its armature into relay-energized position. Condenser lit therefore permits an initial heavy surge of energizing current in the winding of relay 2. Although the condenser charges and the relay current is limited by resistor i i2 at a lower value, this does not cause release of relay X, as the holding current of a relay is considerably less than the initial pull in current thereof.

The following is a table of preferred or normal values for the more importantelements ofV the circuit in a typical `system for application vin a largebomber airplane:

Element Normal Slow Release P eriod lRelay L 1.5 seconds. Relay V 0.37 second.

0.16 second (inherently). Relay W..v 0.6 second (Prolonged by Condenser 58). Relay X Negligible Relay Y 0.35 second. Relay Z 2.75 seconds.

Value Condenser 52 250 nifd. Resistor 49 '1,000 ohms. Condenser S8.. 100 mid. Resistors V'3.4 and 95 200 ohms total. Condenser 58.. 100 mfd. Resistor 108...- 60 ohms Resistor 112 100 ohms Condenser 114. '60 mid Resistor 100 r500 ,ohms Condenser 102 mid. Resistor 36 2,000 ohms Condenser 38 500 rnfd.

Pre-.Landing Condition As Vthe airplane approaches the landing strip the master switch I8 will be closed to apply voltage to conductor 20. This also energizes conductor 2G, as with the airplane still oid the ground, the landing gear switch 2d is then closed. Moreover, as the landing Wheel has not yet rotated, the skid-and-recovery switch 6I is in neutral position with its contacts S and AR. both open, andcommutator $6 is stationary.

Under such .conditions relay L is energized through rectier de and conductors 42 and fil; and because relay V is then unenergized, an energizing circuit for relay W is formed through conductor 2), rectier 654, conductor 55, relay contact Vb, conductorV 54 and relay contact La, connected in series to ground at 48. Relay X is then energized directly through relay contact Wa, its energizing current passing through resistors mii and H2. Upon energization of relay W, relay Y, which had been energized Ythrough relay contact Wb and conductors 96 and 98, connected in series therewith, now is deenergized. This results in release of the switch arm of relay Y for movement against contact Yb upon expiration of the release delay period of this relay. Relay Z, on the other hand, is energized by current ilow through landing gear switch .24, a branch of conductor 2d, rectifier ,23, a branch of conductor 3c, and conductor 32.

Because relays Z and X are both energized at the time mentioned, the solenoid lli" of solenoid valve lil is energized and therebylmaintains the wheel bra-lies released from hydraulic pressure. Solenoid energizing current flows through relay contact Xe, conductor 4i), relay contact Zo and conductor el. Thus the airplanes wheel brakes remain in released condition during the approach to the .landing strip and there is assurance that the initial ground contact of the wheel will not be accompanied by brake-produced slridding thereof which might go undetected and prove dangerous. It is desirable that the wheel should be permitted to accelerate substantially to running speed before the brakes are applied upon landing initially.

Initial ground Contact of wheel under normal runway conditions The first event occurring in the system upon Contact of the wheel with the ground initially is the opening of landing gear switch 24. Although 10 'this interrupts previous flow of energizing current to relay Z, relay L remains energized through relay `contact Xa. Relay Z does not release immediately, however, because of its slow release lrecovery contacts R and holds them closed until 'after the wheel vhas accelerated substantially rto full ground speed, which usually requires at least a major fraction of a second, if not somewhat longer. It is important for reasons of braking eiciency to allow acceleration of the landing wheel substantially to full running speed before permitting application of the brakes for the rst time during a landing. 'Ifo accomplish this result relay X, hence relay W, must remain energized until recovery contacts R again open. Relay W is initially energized through a circuit including rectifier Se, conductor 5E, relay Contact Vb, conductor 54 and relay contact Le, but as soon `as the landing wheel begins to rotate upon contact with the groundJ attendant rotation of commutator 66 eects energization .of relay V and thereby opens said energizing circuit for relay W. Relay W does not release immediately, however, having a slow release period extended to 0.6 second by condenser 53 connected across its field coil. Before 'relay W can release, closure of recovery contacts R accompanying wheel acceleration forms a new energizing circuit for relay W through relay contact Yb and conductors 60 and IEM. "Ihusrelay W remains in its energized position continuously during the entire period of wheel acceleration. Rectifier t@ is of such polar- -ity that it `permits condenser 58 to prolong the slow release period of relay W after energization through relay `contact Vb but not after energization thereof through conductor 6&3,

When the wheel ceases to accelerate further, having attained substantially full running speed, recovery contacts R automatically open, thereby breaking the energizing circuit for relay W. When relay W releases, so does relay X, which interrupts the vsolenoid energizing circuit at Xc. The solenoid valve IG is thereby actuated :for brake application.

As a result of the foregoing it will be seen that the landing wheel is allowed to attain substantially full rotative speed after initial ground contact, before the system energizes solenoid Ill' to permit application of Vthe brakes. If for 4any reason the system otherwise fails to deenergize the valve solenoid lli before expiration of the release period of fail-safe relay Z, which is normally about 2.75 seconds, after landing gear switch 24 opens, relay Z will release of its own accord and break the valve solenoid energizing circuit at relay contact Zc. This would be true of fail-safe relay Z should a system failure occur, such as, for example, sticking of the contacts of either of relays W or X in relay-energized position.

Skid-preventing operation laftci initial brake application When the landing wheel approaches full rol l Vt'a'tive speed after initial contact with the ground and the recovery contacts R have closed and then opened to apply the brakes, the wheel soon'cominences to skid ii brake pressure is eXcessive as it will ordinarily be on any runway during the initial ground contact period when the weight load on the wheel is still very light. The condition of the circuit at the time such skidding begins to develop is the following: landing gear switch 265 is open, relays L, W and X are released, relays V, Y and Z are energized and contacts R and. S are open. When the wheel decelerates at a suiiiciently rapid rate the skid contacts S close thereby energizing relay W which in turn energizes relay X. The latter then forms an energizing circuit for solenoid lil through relay contacts Xe anqd Ze, to release the brakes once again so that the wheel may recover from the skid.

With the energica-tion of relays W and X as just described, relays Y and Z are deenergized,

and if they remain so will release automatically after about 0.35 second and 1.5 seconds, respec tively.

Upon interruption of wheel skidding by energization or" the valve solenoid le', the wheel starts to accelerate. On a slippery runway, or if the Weight borne by the wheel is still relatively light, the wheel will accelerate at a slow rate. On the other hand, if the wheel carries a substantial weight load and is rolling on a runway having high coeiiicient of friction, it will accelerate very rapidly. The system operates in one way under one oi these circumstances and in a diierent way under the other.

YIf the wheel accelerates rapidly, that is, at such a rate that the recovery contacts R are closed within a period of time equal to the slow release period of relay Y (about 6.35 second) after a skid signal effects deenergization of such relay, such recovery signal will immediately restore pressure to the brakes by deenergizing the solenoid IS. This is accomplished by grounding the previously energized side of the coil of relay X through conductor H36, relay contact Ya, conductor llll and closed recovery contacts R. Short circuit current through relay contact Wa caused by such closure of recovery contacts R is limited by resistor l @3.

The nature of the wheel skid-and-recovery switch mechanism (Si) determining the manner in which the contacts S and R thereof are opened and closed in response to wheel acceleration and deceleration, is set forth in my above cited copending application Serial No. 90,965 led May 2, 1949. ln accordance with the operation of such mechanism, rapid acceleration of the landing wheel after release of the brakes causes corre spondingly quick opening of skid contacts S and closing of recovery contacts R, producing the effect mentioned in the preceding paragraph. On the other hand, such switch mechanism, including a flywheel and an overrunning clutch type of device acting cooperatively on the switch actu ating structure in the mechanism, is such that the skid-sensing contacts S will remain closed for an extended period of time after release of the brakes if the wheel accelerates at a relatively slow rate. The reason this is true is that a considerable length of time is required for the slowly accelerating wheel to increase in speed above the speed of the slowly decelerating iiywheel in order to open the skid contacts S and cause closure of the recovery contacts R. The normal slow release period of relay W acts as a time 'bridge' preventing release of this relay in the short interval between such opening of skid contacts S and ensuing closure oi recovery contacts R.

' Thus, in the case of a relatively slippery runway or Va light load on the landing wheel, the skid contacts S are held closed for a-n extended period. Such prolonged closure oi skid contacts S maintains energization or relay W, hence deonergization of the valve solenoid lil to keep the wheel brakes in the ofi condition and allow the wheel to continue to accelerate, however slowly. When nally recovery contacts R do close, they take the place oi the skid contacts S to maintain the relay W energized and the brakes released as in the previously existing condition. Thus the wheel is permitted to continue its acceleration towards full rotative speed even though skid contactsS have opened and recovery con-- tacts R close. When the wheel does approach full rotative speed, the recovery contacts R will finally open and thereby break the energizing circuit for relay W to cause deenergization of relay X after the short slow release period of relay W (0.15 second). Release of relay X opens the energizing circuit for solenoid I6 at relay contact Xe, and the brakes are applied.

in the presently improved system, therefore, the reapplication of brakes by deenergization of solenoid ill occurs by one mode of operation of the circuit components including the skid-andrecovery switch mechanism 6i under vconditions ci rapid wheel acceleration following skid-preventing brake release, and by a diierent mode of operation of its components under conditions of relatively slow acceleration of the wheel. If, under the condition of rapid wheel acceleration, the recovery contacts R close before the delayed release 0f relay Y, such closure of recovery con-- tacts immediately deenergizes relay X and reapplies the brakes. However, if closure of recovery contacts Recours after release of relay Y, the brakes will not be released at that time, but later, namely when the recovery contacts R again open. Thus the slow release period of relay Y is a cri= terion. or reference in the system by which it is possible for the apparatus to time the rapidity oi reapplication oi the brakes after a skid in accordance with landing conditions prevailing at the instant. II" the wheel accelerates quickly after release of the brakes following a skid, then it is equally able to, and does, receive an immediate reapplication of the brakes. i However, if the wheel accelerates slowly after release of the brakes following a skid, then the brakes are not reapplied until after a measured or prolonged interval during which the wheel is permitted to attain full rotative speed.

The function or" relay Z in the present improved apparatus is separate from the modes of operation just described. Ils only purpose is to insure that the brakes will be reapplied in any case after a period of about 2.75 seconds during energization of relay W. Thus, for example, relay Z will release automatically at the end of such period should the contacts of relay W become stuck in the lower or Wa position.

It will be understood, of course, that as the airplane continues down the runway during landing, the skid and recovery contacts S and R may close and open intermittently to cause recycling of the apparatus, hence pulsating braking effort. The overall eect of such operation is generally as described in the copending application Serial No. 145,368 cited above. The apparatus maintains high effective braking forcel on the Wheel by preventing the latter from entering a prolonged skid and by restoring brake pressure as soon as feasible following removal thereof to interrupt the development oi a skid. Certain details of the presently improved system differ from those disclosed in copending application Serial No. 145,368 with respect to the normal skid-preventing operations oi the two systems, but they are fundamentally alike in general principles and. objectives with reference to such operations thereof.

Operation in event of bounce landing Under the caption P'relandingr condition above herein,l it was pointed out that the landing wheels should be allowed to attain substantially full rotative speed after their initial contact with the ground before the brakes are applied. One reason for this is that it enables braking to be carried out most eihciently from the outset. Another reason is that it prevents the possibility of landing with brake-locked wheels. which would cause the wheels to skid from the very beginning and thereby prevent the automatic skid-detecting apparatus from beine conditioned for operation by initial spinning of the reference lywheel in the skid-detecting mechanism described in my earlier application Serial No. 96,965.

It will be evident that a similar problem can arise in the case of a faulty landing in which a Wheel leaves the ground after initial contact therewith and then resumes ground contact at a later time during the landing run. The diniculty here is that such initial ground contact of the wheel spins it fast enough to condition the striddetecting apparatus for operation as in a normal landing, and, unless otherwise prevented, may permit application of the brakes before or at second ground contact of the wheel.

Thev presently improved system insures that the brakes will be in released lposition at the endof a bounce interval no matter how long that iii-- terval may be, so that brake operation will be the same on the second or renewed ground contact of the wheel as in the case of landing initially. This result is accomplished by the combined action f the skid-detecting mechanism, including skid contacts S, and that portion of the system including commutator te, locked wheel control relay V, arming relay L and landing gear switch 2Q.

More particularly, during a bounce landing the landing wheel is spun upon first contact with the ground. This results in. spinning of the lywheel in the skid-detecting mechanism described in application Serial- No. 99,965 cited above. When the spinning wheel leaves the ground it decelerates at a faster rate than does the ilywheel, resulting in closure of skid contacts S. The latter, remaining closed substantially until the flywheel iinally stops spinning, energizes relay W' and thereby brake-releasing relay X. Thus if the wheel contacts the ground a second timey in` the period before the ilywheel stops spinning the brakes will be in released position, as desired. Should the bounce interval be longer, so tha-t theI wheel contacts the ground for the secondl time after the flywheel stops spinning, skid contacts S will then be open. However, relay W is energized nevertheless, as When the landing wheel stops spinning, which it does before the ywheel, relay V is deenergised because commutator St no longer rotates. Landing gearl switch Edi beingy closed, relay L is energized ;A and relay W now receives energizing current through relay contacts Vb and La.

Thus no matter how long the bounce interval may be the brakes will be released when the landing wheel again makes ground contact, relay W being energized either through skid contacts S or relay contacts Vb and La, depending on the length of such interval. The system thereby automatically insures that the braces will bein released condition not only upon initial ground contact but also upon ensuing ground contacts caused by bounce or equivalent faulty landings.

Locked wheel prevention on very icy runway A chief purpose of the commutator 66, relay V andl bridge rectifier circuit l et seq., in addition to preventing brakedwheel condition during initial ground contact and during bounce landings, is to prevent a similar condition on eX- tremely slippery surfaces. As long as the commutator continues to rotate, pulses of energizing current flow through the winding or relay V.

Even at very slow speeds of rotation of the wheel,

hence of commutator tti, this relay is energized, although, with a typical relay used for the purpose, and with a four segment commutator as shown, such relay will not remain steadily in its upper position in the drawing but will oscillate up and down when the wheel speed drops below about twenty-one revolutions per minute. Each reversal of current in the rectifier storage condenser 88, caused by switching action of the cominutator t5, either from a conductive segment to a non-conductive segment, or vice versa, produces a pulse of energizing current in relay V. Such energizing current thereby causes interruption of energizing circuit for relay W through rectifier tt and relay Contact Vd, assuming such energizing circuit is not already interrupted by releaseY of relay L. When commutator 56 ceasesI so, the

- ground, energizing current for relay L must necessarily ow through relay contact Xe and conductors 42 and fil'. Thus, if for any reason the wheel should loci; following release of relay X and attendant application of the brakes, and the skid contacts S should fail to close, the resultant stopping of commutator 55 and release of relay V will insure release oi the brakes by energization of relay W.

The condition which would result in a failure of skid contacts S- to close despite a locked wheel duringr the landing roll, is very extreme and would occur only during the early portion of a landing run, when the wheel load is still relatively light and on a runway or" a slipperiness having ice and a small accumulation of wate on top of the ice. Nevertheless it is a possibility andthe system is devised to protect against any dangerous con sequences of a locked wheel under these conditions.

Under the extreme landing conditions just mentioned, when the brakes are applied the wheel will decelerate very rapidly and may ap-V proach zero speed even though the normal running speed may be as highas 3G() R. P. M. or more. Moreover, because of the extreme slips periness of the runway the wheel is very slow toaccelerate following release of the brakes` efiected by closure of skid contacts S,l so slow that it fails to close recovery contacts R at any time during the acceleration period. However, the

skid contacts S remain closed for an extended period because of the slow rate of acceleration of the wheel, until finally the skid contacts themselves open and the switch mechanism assumes a neutral position with neither contacts S nor the contacts R closed. In approximately 0.15 second after opening oi skid contacts S relay W releases, thereupon releasing relay X and causing reapplication of the brakes by deenergizing solenoid it. Because of the extremely slow rate of acceleration of the landing wheel leading up to opening of sliid contacts S the flywheel speed has dropped considerabl:J by that time. Thus the accelerating landing wheel has come only a fraction of the way up to its normal running speed by the time it overruns the iiywheel and opens the skid contacts S to reapply the brakes.

With such reapplication of the braises the Wheel is again decelerated very rapidly to nearly zero speed. Thereupon the brakes are again released because of closure of skid contacts S, and there again commences a period of very slow acceleration of the wheel, again so slow that it never exceeds in closing recovery contacts R. In the meanwhile the iiywheel in the slzid-and-recovery switch mechanism is continuing to slow down. Thus the slowly accelerating landing wheel overtakes the decelerating flywheel at an even slower speed than it did on the preceding cycle, causing opening or" skid contacts S and reapplication of the brakes approximately 0.15 second later, the release period of relay W. The entire cycle then repeats itself one or more additional times until the speed of the rotating system including the flywheel and the landing wheel has degenerated progressively to the point of substantially zero speed oi rotation. When this condition is attained, the skid contacts S will fail to close as the flywheel has insufhcient speed to operate the switch mechanism when the wheel brakes are again applied. A locked wheel results.

Uponsuch degeneration of braking to the condition of a locked wheel, relay V is released, as previously mentioned. Because of its slow release period of about 1.5 seconds, relay L remains in energized position after release of relay X shortly before. Thus an energizing circuit for relay W is formed through relay contacts Vb and La, releasing the wheel brakes once again and allowing the wheel to accelerate. The instant the wheel begins to rotate commutator S, the latter effects reenergization of relay V and thereby interrupts such energizing circuit for relay W. However, relay W does not immediately release because the delaying action of condenser 58 when added to the natural slow release period of this relay, prevents it from releasing until about 0.6 second after relay V is energized. rlhis amount of delay has been found to be suiiicient in a typical case to start the skid-preventing mechanism on a regenerative braking cycle in which the landing wheel and ywheel speeds build up to nearly full running speed. Such regeneration is aided by the fact that an increasing proportion of the airplanes weight is now being borne by the landing wheel and its traction on even the most slippery runway is rapidly increasing.

Brake-holding action, with airpZane at standstill The chief purpose of arming relay L is to prevent automatic release of the brakes by release of relay V when the airplane comes to a stop. Otherwise the portion of the system including commutator ES, relay V and the bridge rectifier would function in the same manner at that time as it does, for instance, at the end of a degeneratiye braking condition described above. Relay L accomplishes its function by automatically releasing, and thereby opening any energizing circuit for relay W through relay contact Vb, at the expiration of its slow release period (1.5 seconds) following the last skid signal producing energization of relay X. Thus when the airplane is brought to a halt the mere fact that commutator 5t has ceased to rotate will not cause energization of relay W and release of the brakes.

In flight, however, as already explained, the landing gear switch 2li is continuously closed and supplies energizing current to relay L, thereby making it possible for the relay V, in4 released condition, to energize the relay W and maintain the brakes in released condition.

I claim as my invention:

1. In an automatic wheel brake control system, the combination comprising normally inoperative brake-release means operable by actuation thereof to effect release of the brake, automatic' skid-preventing means of a type requiring wheel rotation for the control of such means, operatively connected to said brake-release means for actuating the same in response to incipient skidding of the wheel, and locked-wheel preventing means operatively connected to said brake-release means and controlled by wheel rotation independently of said skid-preventing means, said locked-wheel preventing means operating automatically in response to the condition of a substantially non-rotating Wheel to actuate said brake-release means and thereby prevent wheel lock incapable of detection by said skid-preventing means.

2. The system dened in claim l, and disabling means controlled Iby the skid-preventing means and arranged to prevent operation of the lockedwheel preventing means, said disabling means operating automatically a predetermined time after the final operation of the skid-preventing means to prevent actuation of the brake-release means thereafter by said locked-wheel preventing means, and thereby permit braking of the rotated wheel when the same comes to a standstill.

3. In an automatic airplane landing wheel brake control system, the combination comprising normally inoperative airplane wheel brakerelease means operable by actuation thereof to effect release of the brake, automatic skid-preventing means of a type requiring wheel rotation for the control of such means operatively connected to said brake-release means for actuating the same in response to incipient siridding of the wheel, locked-wheel preventing means operatively connected to said brake-release means and controlled by wheel rotation independently of said skid-preventing means, said locked-wheel preventing means operating automatically in response to the condition of a substantially non-rotating wheel to actuate said brake-release means and thereby prevent wheel lock incapable of detection by said skid-preventing means, arming means controlling said locked-wheel preventing means and controlled by operation of the skidpreyenting means to disable such locked-wheel preventing means automatically a predetermined time following operation of said skid-preventing means to actuate said brake-release means, whereby said locked-wheel preventing means is rendered inoperative with the wheel brought to a standstill, and airplane weight-actuated means controlling said arming means and operating automatically in response to the airplane becoming airborne to prevent operation of said arming means and thereby permit operation of said locked-wheel preventing means until said skidpreventing means has again operated during the next succeeding landing of the airplane.

4. In an airplane automatic wheel brake control system, the combination comprising normally inoperative airplane Wheel brake-release means operable by actuation thereof to effect release of the brake, and locked-Wheel preventing means operatively connected to said brake-release means and controlled by wheel rotation, said locked- Wheel preventing means operating automatically in response to the condition of a substantially non-rotating wheel to actuate said brake-release means and thereby release the brake automatically in response to stopping of Wheel rotation.

5. The automatic wheel brake control system defined in claim 4, and time delay means arranged to be initiated automatically upon resumption of wheel rotation after operation of the locked- Wheel preventing means, said time delay means controlling the brake-release means to delay deactuation thereof, hence reapplication of the brake, for a predetermined interval following such resumption of Wheel rotation.

6. The automatic Wheel brake control system dened in claim 5, and automatic skid-preventing means controlled by Wheel rotation and operatively connected to the brake-release means for actuating the same in response to incipient skidding of the Wheel.

7. The automatic Wheel brake control system defined in claim 6, and arming means controlled by operation of the skid-preventing means and in turn automatically disabling the locked-Wheel preventing means during a period after a predetermined interval of inoperativeness of such skid-preventing means.

8. The automatic wheel brake control system dened in claim 7, and means operable automatically in response to the airplane becoming airborne, and in turn controlling the arming means thereby to prevent disablement of the locked- Wheel preventing means otherwise by reason of inoperativeness of the skid-preventing means with the airplane in the air.

9. The automatic Wheel brake control system r 10. The automatic wheel brake control system dened in claim 9, and fail-safe means controlled by operation of the skid-preventing means and operable to disable the brake-release means automatically after a predetermined interval of inoperativeness of such skid-preventing means, said fail-safe means being rendered inoperative by the airborne-responsive means with the airplane in the air.

11. The automatic brake control system defined in claim 4, wherein the locked-Wheel preventing means further includes electrically actuated means controlling the brake-release means, and the wheel-rotation controlled means comprises electric Vsignal generating means driven by the Wheel and operatively connected to the electrically actuated means for actuating the same by wheel rotation.

12. The automatic brake control system defined in claim 11, wherein the electric signal generating means comprises a Wheel-rotated commutator and energizing circuit means controlled thereby providing energizing current to the electrically actuated means during rotation of said commutator.

13. In an automatic airplane landing Wheel brake control system, the combination comprising normally inoperative airplane wheel brakerelease means operable by actuation thereof to effect release of the brake, automatic skid-prevent-` ing means of a type requiring wheel rotation for the control of such means operatively connected to said brake-release means for actuating the same in response to incipient skidding of the Wheel, and locked-wheel preventing means operatively connected to said brake-release means and controlled by Wheel rotation independently of said skid-preventing means, said locked-wheel preventing means operating automatically in response to the condition of a substantially non-rotating wheel to actuate said brake-release means and thereby prevent wheel lock incapable of detection by said skid-preventing means.

GORDON W. YARBER.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,170,237 Farmer et al. Aug. 22, 1939 2,232,752 Wilson Feb. 25, 1941 2,258,820 Sorenson Oct. 14, 1941 2,426,575 Eksergian Aug. 26, 1947 2,468,199 Hines Aug. A26, 1949 

